People on a pure ketogenic diet that weight lift feel weak because at the end of the week they've depleted all their "glycogen storages" in their muscle tissue. Every time you weight lift (an anaerobic process) you use glycogen as your fuel.

Go read the thread I just posted on Cyclical Ketogenic Dieting. You cycle a "carb load-up" every weekend and that gets you through the week's workouts. I've lost 8lbs this past week on the diet and lifted more in the gym this week.